We have a strict DKIM policy that is "you sign it - we check and enforce it", for failed keys ('pub_key_unavailable' and 'failed_key_import') we defer with a 421 and appropriate message in the hope that the other party will fix their problem(s).

The problem is that ProtoDave.com says 'Success' when parsing Dell's key:

SELECTOR Selectors <http://www.dkim.org/info/dkim-faq.html#technical>enable a single domain to have multiple keys. Some domains, like Twitter and eBay, use “*dkim*”. Google Apps domains typically use “*google*”. Others simply use “*default*”. Enter yours here. (Note: Do not include “_domainkey”)